In Manhattan federal court papers, Gerson claims she used a coupon, good for $5 off a $50-or-

more purchase during a Jan. 10 shopping spree at Century 21’s Paramus, NJ, store.

Gerson plunked down a total of $106.82, before tax, for items that included faux-suede moccasins, a child’s dress, stuffed toy and little girls’ lace tights, underwear and T-shirts, according to the filing.

But then, on Jan. 19, Gerson returned a $17.97 kid’s ensemble, claiming it was the wrong color, the filing states. Century 21 refunded her money — minus the 80 cents she’d saved, on a pro-rated basis, by using the discount coupon.

Gerson, a hat designer who sells her creations from her suburban home, flipped her lid.

Century 21’s take-back, she claims, deprived her of the “full benefit” of the $5-off coupon.

Gerson argued that if she’d known she’d lose coupon credit on returned items, she would have purchased them separately.

The class-action suit, which names no other plaintiffs, seeks at least $5 million, and contends that “hundreds or thousands” of other shoppers could also be hurt by the store’s “fraudulent . . . [and] misleading” promotion.

The store did not immediately return a call for comment.

Gerson’s suit was filed by her lawyer dad, Harry Katz, who has a history of going after retailers in federal court.

Dick’s Sporting Goods, Casual Male and The Children’s Place are among those sued by Katz over alleged coupon “schemes.” Those suits were either dismissed or settled.

Asked for comment at her home yesterday, Gerson said, “No, thank you.”